Question

I have an application where I need to write a new getpid function to replace the original one of the OS. The implementation would be similar to:

pid_t getpid(void) {     if (gi_PID != -1)     {         return gi_PID;     }     else     {         // OS level getpid() function     } } 

How can I call the original getpid() implementation of the OS through this function?

EDIT: I tried:

pid_t getpid(void) {     if (gi_PID != -1)     {         return gi_PID;     }     else     {         return _getpid();     } } 

as Jonathan has suggested. This gave me the following errors when compiling with g++:

In function pid_t getpid()': SerendibPlugin.cpp:882: error: _getpid’ undeclared (first use this function) SerendibPlugin.cpp:882: error: (Each undeclared identifier is reported only once for each function it appears in.)

EDIT 2: I’ve managed to get this to work by using a function pointer and setting it to the next second symbol with the id ‘getpid’, using dlsym(RTLD_NEXT, ‘getpid’).

Here’s my sample code:

vi xx.c 'xx.c' 23 lines, 425 characters  #include <sys/types.h> #include <unistd.h> #include <stdio.h> #include <iostream> #include <dlfcn.h>  using namespace std; pid_t(*___getpid)();  pid_t getpid(void) {     cout << 'My getpid' << endl;     cout << 'PID :' << (*___getpid)() << endl;     return (*___getpid)(); }  int main(void) {     ___getpid = (pid_t(*)())dlsym(RTLD_NEXT, 'getpid');     pid_t p1 = getpid();     printf('%d \n', (int)p1);     return(0); } 

g++ xx.c -o xout

My getpid PID :7802 7802  

Answer 1

On many systems, you will find that getpid() is a ‘weak symbol’ for _getpid(), which can be called in lieu of getpid().

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The first version of the answer mentioned __getpid(); the mention was removed swiftly since it was erroneous.

This code works for me on Solaris 10 (SPARC) – with a C++ compiler:

#include <sys/types.h> #include <unistd.h> #include <stdio.h>  extern 'C' pid_t _getpid();  pid_t getpid(void) {     return(-1); }  int main(void) {     pid_t p1 = getpid();     pid_t p2 = _getpid();     printf('%d vs %d\n', (int)p1, (int)p2);     return(0); } 

This code works for me on Solaris 10 (SPARC) – with a C compiler:

Black JL: cat xx.c #include <sys/types.h> #include <unistd.h> #include <stdio.h>  pid_t getpid(void) {     return(-1); }  int main(void) {     pid_t p1 = getpid();     pid_t p2 = _getpid();     printf('%d vs %d\n', (int)p1, (int)p2);     return(0); } Black JL: make xx && ./xx cc     xx.c   -o xx 'xx.c', line 13: warning: implicit function declaration: _getpid -1 vs 29808 Black JL: